High-frequency phase measuring apparatus



Feb. 25, 1947. A w. w. HANSIEN E-rAL 2,416,310

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THEIR ATTORNEY.

Feb. 25, 1947. 1

w.'w.HANsE ETAL 2,416,310

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Patented Feb. 1947 UNITED STATES PATENT OFFICE man-rasqunucr muss MEASURING mans'rus William W. Hansen, Garden City, and Morris Rels'on, Kew Gardens, N. Y., assignors to Sperry Gyroscope Company, Inc., Brooklyn, N. Y., a corporation of New York Application November 5, 1942, Serial no. 464,613 22 Claims. (01. 112-245) The present invention relates to the art ineluding phase measuring circuits-especially for high radio frequencies.

In many instances it is necessary or desirable to measure the phase delay introduced into a circuit by a particular piece of apparatus at high It may also be desirable to" with respect to the measuring apparatus.

By the present invention an improved phase shift measuring circuit is provided adapted for operation with radio frequencies which may be varied over a. wide range. This is done in the present case by converting the phase shift of the high variable radio frequency to an equivalent phase shift of a low constant frequency which is thereafter measured.

The low frequency may range in which phase measurement is quite simple, such as in the medium audio frequency range. This low frequency is made independent of the high radio frequency at which measurements are to be made and, accordingly, the entire device is rendered substantially independent of the value of the radio frequency used for the test.

Accordingly, it is anobiect of the present in.- vention to provide an improved method for measuring phase-shift and an improved phase-shift measuring circuit adapted for use in measuring be selected within a the phase shift of circuit apparatus at high radio I frequencies.

It is another object of the present invention to provide improved phase shift measuring apparatus suitable for use in measuring the phase shift of circuit apparatus with varying high radio frequencies, whereby the phase shift versus frequency characteristics of such apparatus may be easily determined.

It is still another object of the present invention to provide improved high frequency phaseshift measuring apparatus in which the high frequency phase shift to be measured is converted by an improved means to a corresponding phase shift at a fixed low frequency independent thereof.

Other objects and advantages of the present invention will become apparent from the following specification and drawings, wherein,

' Fig. 1 shows a schematic block diagram of one embodiment of the present invention,

Figs. 2 and 3 show vector diagrams useful in explaining the operation of the invention,

Fig. 4 shows a schematic block diagramof a modified form of the invention, and

Figs. 5, 6 and ,7 show schematic block dia-.

grams of further embodiments of the invention.

' Fig. 8 is an additional vector diagram useful in explaining the operation of the invention.

Referring now to Fig. 1, there is provided an oscillator I l which may be any suitable source of high-radio frequency oscillations of frequency jo at which frequency the phase shift of the apparatus under test, such as indicated by the block l2,is to be measured. Assuming that the output voltage of oscillator I I may be instantaneously represented by the expression cos wot, this wave is passed through the test apparatus l2 and will appear in the output thereof as a wave having the form-cos (wo being delayed in phase therein by the amount #5 which is to be measured.

There is also provided a further oscillator ii of any suitable type, preferably of a lowfrequen- .cy f1 which may be in the intermediate audio frequency range or of any other value adapted for simple measurement of phase or amplitude. The output wave of oscillator l3 will then have the form cos wit. This output wave is modulated upon either the output wave of oscillator I l or of test apparatus I2 in a balanced modulator H, which, as is well known, serves to suppress the carrier frequency corresponding to the frequency Io of oscillator Il.' Any conventional balanced modulator or carrier-suppressing modulator may be used here. If, for illustration, the output of oscillator II is so modulated, the output wave from balanced modulator M will have the wave shape cos (wit) -cos (out), and this wave is additively combined with the wave output from test apparatus I2 in a suitable combining circuit l6 to produce in its output a wave having the form cos (wot--) +cos (out) cos (wot) (1) Combining circuit It may be any suitable circuit.

the rectified wave exceeding the frequency [1 of oscillator It.

' 1 device l7.

1 cos (wot-) +2 cos (wot-1p) cos (wit) cos twmD i-cos (wit) cos (eat) (2) It Wm be as... thatfthe first and last terms of expression (2) contain both components hav-' ing frequencies of the order of 2,70 and direct our- 1 rent components, both of which are suppressed by the filter portion of circuit H. The second term 1 of expression (2) may; by trigonometrical manipuj lation', be shown to be. equal to:

cos 'cos (wit) and this wave will be passed to the output of This output then comprises a wave of frequency f1 having amplitude proportional to q the cosine of the phase shift produced in the test apparatus I 2. Accordingly, to measure this 1 phase shift it is merely necessary to provide an resultant of the three vectors, 22, 23 and 2i,

namely, the vector R, no longer has a constant phase angle with respect to the carrier vector indicator, such as 1.8, adapted'to indicate the f amplitude of a wave of frequency ii. If desired, 1 this indicator may be directly calibrated to indicate phase shift As an alternative measuring and indicating circuit, the output of combining circuit it may be fed'to a frequency modulation detector and filter 3 iii. For an explanation of the operation of this detector [9, reference is first made to Figs. 2 and 3.

Fig. 2 shows a vector diagram of an ordinary 1 carrier wave'2l, amplitude modulated (without j 23, which is equal in length to vector 22 but rotates at an angular velocity wo-l-wi. It will therefore a beseen that the resultant R of these three vectors, 1 representing the modulated wave, will be a vector along the direction of vector 2i periodically'varying in magnitude at the frequency of modulation corresponding to the angular velocity oi. The instantaneous magnitude of the modulated '(with- ;out carriersuppression) wave is then the length of this resultant vector R- which lies along the vertical axis 20.

Referring to Fig. 2, the vector 2!, representing the carrier, has a fixed length 0A. However, the vector 25, representing the resultant of the sidebands, has a, length which varies between the length OA and +OA, and the locus of the itip of the vector 2| is a line AA. The resultant vector R is fixed in direction and varies in length between zero and OB, which is twice 0A, and g1; locus of the tip 01' the vector R is the line' In the case of a frequency modulated wave, however, the vector representing the carrier is displaced 90 degrees from the resultant of the isi'deband vectors. In Fig. 3, illustrating frequency modulation, the sideband vectors 22 and 123 are shown, for the sake of comparison, in the must-be represented by a vector 2 i which is perpendicular to the positionin which vector 2!, of Fig. 2, is shown. The vector 2! is thus perpendicular to the sideband resultant 2a The 2 I or with respect to the resultant sideband vector 25 as was the case in Fig. 2, but varies periodically in phase with respect to the fixed angle vectors 2i and 25, at a rate determined by the modulation frequency f1. Such a wave, when passed through a frequency modulation detector such as thedevice i9, will produce in the detector output a wave having the frequency 11.

As in the case of Fig. 2, the vector 25 has a fixed direction and a constantly varying length. The vector 2! also has a fixed direction and a fixed length. As the length of the vector 25 varies, the length of the resultant R also varies, but the angular position thereof varies as well. The icons of the tip of the vector R is a line CD parallel to the line 0A, and having twice the length of 0A.

Amplitude modulation detectors such as the device i'i are so constructed as to be responsive to in-phase variations in magnitude, as repre- 1 sented by Fig. 2, and frequency modulation de= tector, such as represented by the device i9, are so constructed as to be responsive to angular variations, as represented in Fig. 3, but frequency modulation detectors are unresponsive to variations in magnitude.

Referring to Fig. 1, showing an arrangement in which the carrier wave is suppressed by the balanced modulator it, it will be observed, that if there is no phase shift in the test apparatus i2 the output of the test apparatus IE will be in phase with the oscillator H. Since the output of the oscillator H is the carrier wave.'the effect will be the equivalent of reintroducing the carrier wave in the output of the combining circuit It. The output of the oscillator ii may be represented by the vector 2!, and the output of the balanced modulator it may be' represented by the vector 25. Ifthese two vectors are combined in the combining circuit E8, the output may be represented by the resultant R, as shown in Fig. 2.

Ordinarily, however, some phase shift takes place in the test wave apparatus I2, and the output thereof, which may be represented by a vector 24 is neither in phase nor 90 degrees out of phase with the sideband resultant vector 25, but at an intermediate angle thereto, as represented in Fig. 8. Since the combining circuit [6 combines the combining circuit l6, therefore, represents a wave having both amplitude and frequency modu- 1 "same positionas in Fig. 2. However, the carrier tion or variation magnitude, and a frequency modulation detector, such as the device [9, is responsive only to angular variation independent of magnitude or length of a vector, the vector 24, as shown in Fig. 8, may be resolved into two carrier components 30 and 30', respectively, inphase and in quadrature with the vector 25, repre quency inputs. Alternatively, the operating frequency of detector I! may be gauged with that of oscillator ll to .vary together.

vector 2! to combine with the resultant 2' of the 5 sideband vectors 22 and 22 to produce an output which is measured in the indicator it. The

in-phase component 4| bears a ratio of cos c to A the output 24 of the test apparatus l2, and a proportionate output is measured by the indi- 1o cator l8.

On the other hand the component 24' of the vector 24 utilized by the frequency modulation detector I9 is smaller than the vector 24 in the ratio sine 4, and the measured output is reduced in proportion. Accordingly, the frequency-modulation detector I! will have an output wave shape sin 1 sin Nit, which may be indicated in another "indicator 26, similar to indicator ll, which may also be calibrated directly in .terms of the phase shift 4 It is to be understood that any, suitable types of detectors may be used as detectors l1 and II. including those of the superheterodyne type.

It will be seen that the measuring circuit of Fig. l is rendered substantially independent of the frequency is, since this frequency has no eflect whatever upon the resultant inputs to indicators III or 26. Accordingly, this frequency In may be varied over a complete range, if desired, and the corresponding phase shifts directly read of! from indicators is or 28.

In addition, it will be observed that the measurement of the phase shift of a high radio frequency wave has been converted into the measurement of the amplitude of a low fixed frequency wave. Both of these points are advantages in the present system hitherto unrealizable in practice.

Fig. 4 shows a further modification of the sys- 40 tern of Fig. 1 and similar. elements have been denoted by similar reference characters. In Fig. 4

the circuit is exactly the same as ,in Fig. 1 up to the outputs of the amplitude modulation detector l1 and FM detector l9. As was observed above with respect to Fig. 1. these waves have wave shapes respectively represented by cos 4 cos Wit and sin sin flit. These waves are then additively combined in a further combining circuit 21', whose output therefore will have the wave shape cos (mt-qi) Accordingly, in this manner the phase shift it of the wave at the high radio frequency In has been converted into an identical phase shift of the wave of low frequency 11.

This phase shift 4: may then be indicated or measured by a suitable low frequency phase meter 28 which maybe of any type well known in the art.

Accordingly, by this modification the measurement of the phase shift of a high radio frequency go wave has been converted into a measurement cfphase shift at a low frequency which may be easily and accurately performed in any well known manner.

It will be noted that in the aboveFigures 1 6B and 4, the inputs to the AM detector l1 andFM detector l9 are both at substantially the frequency In, which is usually variable over considerable range. The usual type of square law AM with widely varying frequency inputs. However. the usual type of FM detector-requires a single frequency input and accordingly the detector is of the above figures must be of the tmladapted to operate satisfactorily with widely v y l Iredetector is well adapted to operate satisfactorily of Fig. 5. In this instance, the circuit is substantially the same as that of Fig. 4, with the added elements to be described. Thus, an intermediate frequency oscillator 31 is provided having a fixed frequency fa, which is preferably separated from the usual values of the high frequency in at' which measurements are to be made by a substantial frequency difference. The output of oscillator 3| then will have a wave shape cos (mt) This is combined with the output of oscillator ll, having wave shape cos (act), in a suitable frequency converter 32 of any conventional type adapted to produce in its output a wave having a frequency equal to the difference between the frequencies of the waves supplied thereto. Accordingly, the output of frequency converter 32 will have the wave shape cos (mo-(mt; This latter wave is then combined with the output of combining circuit i8, obtained in the same manner as in Figs. 1 and 4, in a furthersirnilar frequency converter 33. Since the output of combining circuit It has a wave shape of the form cos (wot-q) +cos (wit) cos (wot), it will be seen that the output of the frequency converter 33 will then have the wave shape cos (mt-e) +cos (wit) cos (mt) This latter wave shape will be seen to constitute the same relative component as formerly supplied to the detectors I1 and IS in Figs. 1 and 4, but now referred to the fixed intermediate frequency {2 instead of variable high frequency In. Accordingly, the detectors l1 and It may now be constructed to operate at a single fixed input frequency fa, whereby their efficiency, simplicity and general characteristics may be greatly improved. The outputs of detectors l1 and I! are then utilized in the same manner as inulator l4 into corresponding waves at the fre- 1 quency H, which are thereupon combined in the combining circuit l6 and supplied to the detectors l1 and it to be utilized in the same manner as in the previous modifications.

It is to be understood that in any of the circuits of Figs. 4, 5 or 6, the outputs of the detectors I1 and it may be utilized separately in the manner shown in Fig. 1, if desired.

Fig. 7 shows another embodiment of the present invention which is somewhat simpler than the preceding forms. In'this instance, the high frequency wave output of oscillator. ll, of frequency I0, is combined with that of the low frequency oscillator l3, having frequency h, in a frequency converter 36 to provide an output therefrom of frequency fo-h. This'wave is then sup- 78 of frequency converter 31 will therefore have converter 37.

Although the above description has heen'con fined to the measurement of the phase shift of a test apparatus, it will be clear that the present ,auaaro cation of the phase shift in the test apparatus l2.

In this instance, the frequency of oscillator f1 cannot be quite so low as in the previous modiflcations, since the difference frequency fo-fr must be sufliciently far removed from, (that is,

must have a large enough frequency difference with respect to) frequency f to permit satisfac tory separation of these frequencies by means of frequency converter 30, and to permit satisfactory separation-of the frequency f1 from the remaining frequencies in and fo-h in frequency system may also be used to measure radio frequency phase differences. Thus, in all-the figures, oscillator H and test apparatus l2 may be eliminated, and their outputs replaced by two waves.

simply by varyingvthe frequency of oscillator- H over the desired range.

v It will be clear that a suitable recorder may be utilized instead of indicator 28, whereby the actual frequency versus phase characteristics of the test apparatus l2 may be directly recorded.

As many changes could be made in the above construction and many apparently widely different embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense. a What is claimed is:

quency phase shift produced-by test apparatus.

comprising a source of adjustable high frequency wave, means for energiaing said test apparatus from said source of high frequency to produce an output high frequency wave of the same frequency as the output wave of said source but phase-shifted with respect to the output wave of said source by an amount to be' measured, a

source of low frequency wave, means for modulating one of said high frequency waves'by saidlow frequency wave and for suppressing the carrier frequency component of said resultant modulated wave, means for converting both'said carrier- -suppressed wave and the other of said high frequency waves into corresponding waves at a fixed intermediate frequency, means, for additively combining said converted waves, means re= sponsive to saidcombined wave for producing a wave of low frequencyphase-shifted with respect to said first low frequency wave by said phase shift to be measured, and means for comparing the phases of said low frequency waves to indicate said phase shift, whereby said phase shift may be measured for varying high frequencies.

3. Apparatus formeasuring the high frequency phase'shift produced by test apparatus. comprising a source of adjustable high frequency wave, means for energizing said test apparatus from said source to produce an output high frequency wave of the same frequency as the output wave of said source but phase-shifted with respect to the output wave of said source by an amount to be measured, a source of low frequency wave, means for modulating one of said high frequency .waves by said low frequency wave and for suppressing the carrier frequency component of the resultant modulated wave,

, means for additively combining said suppressed- 1. Apparatus for measuring the high frequency phase shift produced by test apparatus,

comprising a source of adjustable high frequency wave, means for energizing said test apparatus from said source to produce an output wave of the same frequency as the output wave of said ant modulated wave, means for additively combining said suppressed-carrier wave with the other of said high frequency waves, a source of fixed intermediate frequency wave, means for converting said combined wave into a corresponding wave 'at said intermediate frequency, means responsive to said converted wave for producing a wave of said low frequency phase-shifted with respect to said low frequency wave by said phase shift to be measured, and means for comparing the phases of said low frequency waves to indicate said phase shift, whereby said phase shift may be measured for varying high frequencies.

2. Apparatus for measuring the high frecarrier wave with the other of said high frequency waves, means responsive to said combined wave for producing a wave of said low frequency phase-shifted with respect to said first low frequency wave by said phase shift to be measured, and means for comparing the phases of saidolow frequency waves to'indicate said phase shift, whereby said phase shift may be measured for varying high frequencies.

4. Apparatus for measuring the high frequency phase shift produced by test apparatus, comprising a source of high frequency wave, means for applying said wave to said test appa-- ratus to produce an output wave of the same frequency as the output wave of said source but phase-shifted with respect to said first high frequency wave, a source of low frequency wave,

,means jointly responsive to said waves for producing a second low frequency wave having a low frequency phase difference with respect to said low frequency wave equivalent to the high frequency phase shift, and. means for indicating said low frequency phase difference to indicate said high frequency phase shift, whereby said high frequency phase shift may be measured for varying high frequencies.

5. Apparatus for measuring the high frequency-phase shift of test apparatus, comprising a source of high frequency wave, means for applying said wave to said test apparatus to produce an output wave therefrom of the same frequency as the output wave of .said source but 1 phase-shifted with respect to said first high frequency wave, a Source of low frequency wave,

corresponding to said phase shift, and means for separately indicating the amplitude of said iow frequency wave to indicate said phase shift,

6. Apparatus for measuring the phase difference between two adjustable high frequency test waves of substantially equal frequency, comprising a source of fixed low frequency wave, means i for modulating one of said test waves by said low frequency wave and for suppressing the carrier frequency component of theresultant modulated wave, means for combining said suppressed-car- ,rier wave with the other of said test waves, fixed ing said combined wave to produce a detected wave at said low frequency, means for frequency modulation detecting said combined wave to produce another detected wave at said low frequency, means for combining said detected low frequency waves, and means for comparing the phase of said original low frequency wave with that of said combined detected wave to indicate said phase difference at said low frequency, whereby said phase difference may be measured for varying high frequencies.

'7. Apparatus for measuring the phase difference between two adjustable high frequency test waves of substantiallyg eqfiilfrequency, comprising a source of fixed ow frequency wave, 'means for modulating one of said test waves by said low frequency wave and for suppressing the carrier frequency component of the resultant modulated wave, means for converting both said suppressedcarrier wave and the other of said test waves into corresponding waves at a fixed intermediate frequency, means for combining said converted waves, means for amplitude modulation detecting said combined wave to produce a first detected wave, means for frequency modulation detecting said combined wave to produce another detected wave at said low frequency, means for combining said detected low frequency waves, and means for comparing the phase of'said original low frequency wave with that of said combined detected wave to indicate said phase difference at said low frequency, whereby said phase difference may be measured for varying high frequencies.

8. "Apparatus for measuring the phase difference between two adjustable high frequency test waves of substantially equal frequency, comprising a source of fixed low frequencywave, means for modulating one of said test/waves by said low frequency wave and for suppressing the carrier frequency component of the resultant modulated wave, means for combining said suppressedcarrier wave with the other of said test waves, means for amplitude modulation detecting said combined wave to produce a detected wave at said low frequency, means for frequency modulation detecting said combined wave to produce another detected wave at said low frequency, means for electrically combining said detected low frequency waves, and means for comparing the phase of said original low frequency wave with that of said combined detected wave to indicate of said test waves by said low frequency wave and for suppressing the carrier frequency component of the resultant modulated wave, means for combining said suppressed-carrier wave with the other of said test waves, means responsive to said combined wave for producing a wave of said low frequency phase-shifted with respect to said first low frequency wave by an amount equivalent to said phase .difference, and means for comparing the phase of said low frequency waves to indicate said phase difference at said low frequency, whereby said phase difference may be .measured for varying high frequencies.

10; Apparatus for measuring the phase difference between two high frequency test waves of substantially equal frequency, comprising a source of fixed low frequency wave, means jointly responsive to said waves for producing a second low frequency wave having an equivalent phase difference at said low frequency, and means for indicating said low frequency phase difference to thereby determine said high frequency phase difference, whereby said phase difference may be measured for varying high frequencies.

11. Apparatus for measuring the phase diiference between two high frequency test waves of substantially equal frequency, comprising a source of low frequency wave, means for modulating one of said test waves by said low frequency wave and for suppressing the. carrier frequency component. of the resultant modulated wave, means for combining said suppressed-carrier wave with the other of said test waves, means for amplitude modulation detecting said combined wave,,and means for separately indicating the amplitude of-said detected wave to thereby indicate said phase difference, whereby said apparatus is adapted to indicate said phase diflerof said test waves by said low frequency wave said phase difference at said low frequency,

whereby said phase difference may be measured at varying high frequencies.

9. Apparatus for measuring the phase difference between two high frequency test waves of substantially equal frequency, comprising a source of low frequency wave, means for modulating one and for suppressing the carrier frequency component of the resultant modulated wave, means for combining said suppressed-carrier wave with the other of said test waves, means for frequency modulation detecting said combined wave, and

means for separately indicating the amplitude of said detected wave to thereby indicate said phase difference, whereby said apparatus is adapted to indicate said phase difference for varying high frequencies.

13. Apparatus for measuring the phase difi'er- I ence between two high frequency test waves of substantially equal frequency, comprising a, source of low frequency wave, means for modulating one of said test waves by said low frequency wave and for suppressing the carrier frequency component of the resultant modulated wave, means for combining said suppressed-carrier wave with said second test wave, means for detecting. said combined wave and producing a low frequency wave having the frequency of said modulation, and means responsive to phase difference between said low frequency waves, whereby said phase difference may be measured for varying quencies. M

14. Apparatus for measuring the phase difference between first and second highfrequency test waves of substantially the same frequency, comprising a source of low frequency wave, means for combining the low frequency wave and one of the high frequency waves to produce a combined high freaaraam f output, means for combining said combined outsource of intermediate frequency wave, means for modulating the first high frequency wave with the low frequency wave to produce a modulated output. means for combining the modulated output with the second high frequency wave, means for heterodyliing the first high frequency 'wave with the intermediate frequency wave,

means for heterodyning the heterodyne output tecting the last combination, and means for com-.

with the combined wave output, means for deparlng the low frequency detected output in phase with the first low frequency wave to indicate the phase relationship between said high frequency wavess 16. Apparatus for measuring the phase diil'erence between first and second high frequency test waves having substantially the same frequency, comprising a source of low frequency wave, a source of intermediate frequency wave, means for modulating the first high frequency wave with the low frequency wave to produce a modulated wave, means for heterodyning the first high frequency wave to produce a first difference wave,

and means for heterodyning and combining said,

first difference wave, said second high frequency wave, and said modulated wave to produce a combined output with means for deriving therefrom a low frequency having a phase relationship with respect to said first low frequency wave indicativeof phase relation between said first and second high frequency waves.

17. Apparatus for measuring the phase diflerence between first and second high frequency test waves having substantially the same frequency, comprising a source of low frequency wave, means for modulating the first test wave by the low frequency wave to produce a resultant mod- 7 ulated wave, means for combining the modulated wave with the second test wave, means for demodulating the combined wave to produce a second low frequency wave of the same frequency as said mentioned low frequency wave, and means for comparing the phase relationship ofsaid low frequency waves to produce anindication of said 1 phase relationship between said high frequency waves. I

18. The method of measuring the phase shift of ,test apparatus for variable high frequencies, comprising the steps of applying a variable high frequency input wave to said test apparatus to produce an output wave therefrom of the same frequency as the said input wave, but phase shift- -'ed with respect to said input wave, generating a low frequency wave, modulating said high frequency input wave with said low frequency wave, and eliminating the carrier wave to producesidebands, combining said sidebands with said high frequency output wave, and demodulating the combination of said waves toproduce a second low frequency wave of the same frequency as said first low frequency wave, but having a phase shift quency wave, and comparing the phase relationship between said low frequency waves to obtain an indication of the phase shift of the high fre- 5 quency waves. Y

19. The method of measuring the'phase difference between first and second high frequency waves of substantially equal frequency, comprising the steps of generating a low frequency wave,

modulating one of said high-frequency waves by saidlow frequency wave to produce a modulated wave, suppressing the carrier frequency component of the modulated wave to produce sldebands, additively combining said sidebands with said other test wave, demodulating the cbmbination to produce a second low frequency wave'having the 'same frequency as the first-mentioned low fre-- quency wave and comparing the low frequency waves in phase to obtain an indication of the phase diiference between the two high frequency waves.

20. The method of measuring the phase difference between two variable high frequency waves of substantially equalfrequency, comprising the steps of generating a low frequency wave, m0du- .lating one of said high frequency waves with the low frequency wave, combining the other of said high frequency waves with said modulated wave,

demodulating the combination tov produce a second low frequency wave, and ascertaining the phase relationship between said low frequency waves-to obtain an indication of the phase relationship between the two high frequency waves.

21. The method of measuring the phase diil'erence between two variable-high frequency waves of substantially equal frequency, comprising the steps of generating a low frequency wave, modulating one of said high frequency waves with said 4 low frequency wave, combining the other of said high frequency waves with said modulated wave, demodulating the combination of said waves t0 produce a second low frequency wave having an amplitude variable in response to variations in phase relationship between said high frequency waves and measuring the amplitude of said second low frequency wave.

22. The method of measuring the phase diflerence between two variable high frequency waves having substantially the same frequency, comquency wave to indicate variations in phase relationship between said high frequency waves.

' WILLIAM W. HANSEN.

MORRIS RELSON.

REFERENCES crrnn The following references are of record in the file of this patent: I STATES PATENTS Date Number Name 2,305,614 Goldste'in Dec. 22, 1942 Armstrong Dec. 8, 1936 corresponding to the phase shift of the high freprising the steps of generating a low frequency. 

